How to write source code in a way that makes it possible for user-visible text to be translated.

Languages and Writing Systems

In some cases internationalization is simple, for example, making a US application accessible to Australian or British users may require little more than a few spelling corrections. But to make a US application usable by Japanese users, or a Korean application usable by German users, will require that the software operate not only in different languages, but use different input techniques, character encodings and presentation conventions.

Qt tries to make internationalization as painless as possible for developers. All input widgets and text drawing methods in Qt offer built-in support for all supported languages. The built-in font engine is capable of correctly and attractively rendering text that contains characters from a variety of different writing systems at the same time.

Qt supports most languages in use today, in particular:

All East Asian languages (Chinese, Japanese and Korean)

All Western languages (using Latin script)

Arabic

Cyrillic languages (Russian, Ukrainian, etc.)

Greek

Hebrew

Thai and Lao

All scripts in Unicode 5.1 that do not require special processing

On Windows, Unix/X11 with FontConfig (client side font support) and Qt for Embedded Linux the following languages are also supported:

Bengali

Devanagari

Dhivehi (Thaana)

Gujarati

Gurmukhi

Kannada

Khmer

Malayalam

Myanmar

Syriac

Tamil

Telugu

Tibetan

N'Ko

Many of these writing systems exhibit special features:

Special line breaking behavior. Some of the Asian languages are written without spaces between words. Line breaking can occur either after every character (with exceptions) as in Chinese, Japanese and Korean, or after logical word boundaries as in Thai.

Bidirectional writing. Arabic and Hebrew are written from right to left, except for numbers and embedded English text which is written left to right. The exact behavior is defined in the Unicode Technical Annex #9.

Non-spacing or diacritical marks (accents or umlauts in European languages). Some languages such as Vietnamese make extensive use of these marks and some characters can have more than one mark at the same time to clarify pronunciation.

Ligatures. In special contexts, some pairs of characters get replaced by a combined glyph forming a ligature. Common examples are the fl and fi ligatures used in typesetting US and European books.

Qt tries to take care of all the special features listed above. You usually don't have to worry about these features so long as you use Qt's input widgets (e.g. QLineEdit, QTextEdit, and derived classes) and Qt's display widgets (e.g. QLabel).

Support for these writing systems is transparent to the programmer and completely encapsulated in Qt's text engine. This means that you don't need to have any knowledge about the writing system used in a particular language, except for the following small points:

QPainter::drawText(int x, int y, const QString &str) will always draw the string with its left edge at the position specified with the x, y parameters. This will usually give you left aligned strings. Arabic and Hebrew application strings are usually right aligned, so for these languages use the version of drawText() that takes a QRect since this will align in accordance with the language.

When you write your own text input controls, use QTextLayout. In some languages (e.g. Arabic or languages from the Indian subcontinent), the width and shape of a glyph changes depending on the surrounding characters, which QTextLayout takes into account. Writing input controls usually requires a certain knowledge of the scripts it is going to be used in. Usually the easiest way is to subclass QLineEdit or QTextEdit.

The following sections give some information on the status of the internationalization (i18n) support in Qt. See also the Qt Linguist manual.

Step by Step

Writing cross-platform international software with Qt is a gentle, incremental process. Your software can become internationalized in the following stages:

Use QString for All User-Visible Text

Since QString uses the Unicode 5.1 encoding internally, every language in the world can be processed transparently using familiar text processing operations. Also, since all Qt functions that present text to the user take a QString as a parameter, there is no char * to QString conversion overhead.

Strings that are in "programmer space" (such as QObject names and file format texts) need not use QString; the traditional char * or the QByteArray class will suffice.

You're unlikely to notice that you are using Unicode; QString, and QChar are just like easier versions of the crude const char * and char from traditional C.

Use tr() for All Literal Text

Wherever your program uses "quoted text" for text that will be presented to the user, ensure that it is processed by the QCoreApplication::translate() function. Essentially all that is necessary to achieve this is to use QObject::tr(). For example, assuming the LoginWidget is a subclass of QWidget:

If you need to have translatable text completely outside a function, there are two macros to help: QT_TR_NOOP() and QT_TRANSLATE_NOOP(). They merely mark the text for extraction by the lupdate utility described below. The macros expand to just the text (without the context).

If you disable the const char * to QString automatic conversion by compiling your software with the macro QT_NO_CAST_FROM_ASCII defined, you'll be very likely to catch any strings you are missing. See QString::fromLatin1() for more information. Disabling the conversion can make programming a bit cumbersome.

Use QKeySequence() for Accelerator Values

Accelerator values such as Ctrl+Q or Alt+F need to be translated too. If you hardcode Qt::CTRL + Qt::Key_Q for "quit" in your application, translators won't be able to override it. The correct idiom is

Run lupdate to extract translatable text from the C++ source code of the Qt application, resulting in a message file for translators (a TS file). The utility recognizes the tr() construct and the QT_TR*_NOOP() macros described above and produces TS files (usually one per language).

Provide translations for the source texts in the TS file, using Qt Linguist. Since TS files are in XML format, you can also edit them by hand.

Run lrelease to obtain a light-weight message file (a QM file) from the TS file, suitable only for end use. Think of the TS files as "source files", and QM files as "object files". The translator edits the TS files, but the users of your application only need the QM files. Both kinds of files are platform and locale independent.

Typically, you will repeat these steps for every release of your application. The lupdate utility does its best to reuse the translations from previous releases.

Before you run lupdate, you should prepare a project file. Here's an example project file (.pro file):

When you run lupdate or lrelease, you must give the name of the project file as a command-line argument.

In this example, four exotic languages are supported: Danish, Finnish, Norwegian and Swedish. If you use qmake, you usually don't need an extra project file for lupdate; your qmake project file will work fine once you add the TRANSLATIONS entry.

linguist, lupdate and lrelease are installed in the bin subdirectory of the base directory Qt is installed into. Click Help|Manual in Qt Linguist to access the user's manual; it contains a tutorial to get you started.

Qt itself contains over 400 strings that will also need to be translated into the languages that you are targeting. You will find translation files for French, German and Simplified Chinese in $QTDIR/translations, as well as a template for translating to other languages. (This directory also contains some additional unsupported translations which may be useful.)

Note the use of QLibraryInfo::location() to locate the Qt translations. Developers should request the path to the translations at run-time by passing QLibraryInfo::TranslationsPath to this function instead of using the QTDIR environment variable in their applications.

Support for Encodings

The QTextCodec class and the facilities in QTextStream make it easy to support many input and output encodings for your users' data. When an application starts, the locale of the machine will determine the 8-bit encoding used when dealing with 8-bit data: such as for font selection, text display, 8-bit text I/O, and character input.

The application may occasionally require encodings other than the default local 8-bit encoding. For example, an application in a Cyrillic KOI8-R locale (the de-facto standard locale in Russia) might need to output Cyrillic in the ISO 8859-5 encoding. Code for this would be:

For converting Unicode to local 8-bit encodings, a shortcut is available: the QString::toLocal8Bit() function returns such 8-bit data. Another useful shortcut is QString::toUtf8(), which returns text in the 8-bit UTF-8 encoding: this perfectly preserves Unicode information while looking like plain ASCII if the text is wholly ASCII.

For converting the other way, there are the QString::fromUtf8() and QString::fromLocal8Bit() convenience functions, or the general code, demonstrated by this conversion from ISO 8859-5 Cyrillic to Unicode conversion:

Ideally Unicode I/O should be used as this maximizes the portability of documents between users around the world, but in reality it is useful to support all the appropriate encodings that your users will need to process existing documents. In general, Unicode (UTF-16 or UTF-8) is best for information transferred between arbitrary people, while within a language or national group, a local standard is often more appropriate. The most important encoding to support is the one returned by QTextCodec::codecForLocale(), as this is the one the user is most likely to need for communicating with other people and applications (this is the codec used by local8Bit()).

Qt supports most of the more frequently used encodings natively. For a complete list of supported encodings see the QTextCodec documentation.

In some cases and for less frequently used encodings it may be necessary to write your own QTextCodec subclass. Depending on the urgency, it may be useful to contact Qt's technical support team or ask on the qt-interest mailing list to see if someone else is already working on supporting the encoding.

Localize

Localization is the process of adapting to local conventions, for example presenting dates and times using the locally preferred formats. Such localizations can be accomplished using appropriate tr() strings.

In the example, for the US we would leave the translation of "AMPM" as it is and thereby use the 12-hour clock branch; but in Europe we would translate it as something else and this will make the code use the 24-hour clock branch.

Localizing images is not recommended. Choose clear icons that are appropriate for all localities, rather than relying on local puns or stretched metaphors. The exception is for images of left and right pointing arrows which may need to be reversed for Arabic and Hebrew locales.

Dynamic Translation

Some applications, such as Qt Linguist, must be able to support changes to the user's language settings while they are still running. To make widgets aware of changes to the installed QTranslators, reimplement the widget's changeEvent() function to check whether the event is a LanguageChange event, and update the text displayed by widgets using the tr() function in the usual way. For example:

LanguageChange events are posted when a new translation is installed using the QCoreApplication::installTranslator() function. Additionally, other application components can also force widgets to update themselves by posting LanguageChange events to them.

Translating Non-Qt Classes

It is sometimes necessary to provide internationalization support for strings used in classes that do not inherit QObject or use the Q_OBJECT macro to enable translation features. Since Qt translates strings at run-time based on the class they are associated with and lupdate looks for translatable strings in the source code, non-Qt classes must use mechanisms that also provide this information.

One way to do this is to add translation support to a non-Qt class using the Q_DECLARE_TR_FUNCTIONS() macro; for example:

This provides the class with tr() functions that can be used to translate strings associated with the class, and makes it possible for lupdate to find translatable strings in the source code.

Alternatively, the QCoreApplication::translate() function can be called with a specific context, and this will be recognized by lupdate and Qt Linguist.

System Support

Some of the operating systems and windowing systems that Qt runs on only have limited support for Unicode. The level of support available in the underlying system has some influence on the support that Qt can provide on those platforms, although in general Qt applications need not be too concerned with platform-specific limitations.

Unix/X11

Filesystem conventions such as UTF-8 are under development in some Unix variants. All Qt file functions allow Unicode, but convert filenames to the local 8-bit encoding, as this is the Unix convention (see QFile::setEncodingFunction() to explore alternative encodings).

File I/O defaults to the local 8-bit encoding, with Unicode options in QTextStream.

Many Unix distributions contain only partial support for some locales. For example, if you have a /usr/share/locale/ja_JP.EUC directory, this does not necessarily mean you can display Japanese text; you also need JIS encoded fonts (or Unicode fonts), and the /usr/share/locale/ja_JP.EUC directory needs to be complete. For best results, use complete locales from your system vendor.

Windows

File I/O defaults to Latin1, with Unicode options in QTextStream. Note that some Windows programs do not understand big-endian Unicode text files even though that is the order prescribed by the Unicode Standard in the absence of higher-level protocols.

Unlike programs written with MFC or plain winlib, Qt programs are portable between Windows 98 and Windows NT. You do not need different binaries to support Unicode.

Mac OS X

For details on Mac-specific translation, refer to the Qt/Mac Specific Issues document here.